The characteristics of a CMOS (complementary metal-oxide-semiconductor) image sensor or a CCD (charge coupled device) image sensor and other image input image sensors have been improved. The demand for these image sensors is growing in various application fields such as digital cameras or camera-incorporated cellular phones.
FIG. 33 shows the spectral sensitivity of an image sensor when using an IR ray eliminating filter rays and using various color filters on the silicon photodiodes that constitutes pixels of the image sensor in terms of light detection levels versus wavelength in Angstroms. The line marked R represents the spectral sensitivity of red pixels having a red filter. The line marked G represents the spectral sensitivity of green pixels having a green filter. The line marked B represents the spectral sensitivity of blue pixels having a blue filter. The line marked W represents the spectral sensitivity if there is no color filter. FIG. 33 illustrates that the spectral characteristics of the three primary colors represented by red, green and blue can be realized by transmitting through red, green and blue color filters.
When using color filters of different colors for each pixel in a single-plate color image sensor the quantum efficiency drops due to attenuation. This attenuation is determined by the spectral characteristics of the light source, the photodiode and the color filter. An attenuation of one half to one third may occur for green pixels.
The pixels in an image sensor usually have red, green and blue color filters. An image sensor may have white (W) pixels having no color filter added to improve sensitivity.
FIG. 34 is an output characteristic illustrating the logarithm of the number of signal charges (ordinate) with respect to the logarithm of luminance (abscissa) for pixels of the various colors. The R G, B and W lines have the same meaning as in FIG. 33. The output characteristics for the pixels of red, green, blue and white become straight lines with the same slope. The number of signal charges is saturated at a certain value.
White pixels with a large number of signal charges in the low luminance region can help improve the sensitivity. Since the white pixels are saturated at lower luminance than the red, green and blue pixels with low quantum efficiency in the high luminance region, the dynamic range in the high luminance region is limited.
In an image sensor having such white pixels, the dynamic range DR of the brightness information is from the floor noise level FN to the saturation level of the white pixels. FIG. 34 illustrates this dynamic range as 80 dB.
It is also possible to form an image using only RGB pixels without using white pixels. However, the resolution is poor since each pixel has less information.
As described above, in a conventional image sensor having white pixels, the white pixels can effectively improve the sensitivity in the low luminance region. However, since the white pixels are saturated at a lower luminance than the R, G and B pixels in the high luminance region, the dynamic range is limited.
Solid-state image sensors having pixels of using red, green and blue color filters having a wide dynamic range while maintaining high sensitivity and a high S/N ratio are disclosed in: Japanese Kokai Patent Application No. 2005-328493; International Patent Publication No. 2005/083790; and Japanese Kokai Patent Application No. 2006-217410.
The problem of this invention is the limitation upon the dynamic range in the high luminance region in an image sensor having white pixels.